The operation principle of a semiconductor radiation detector is based on a depleted volume of semiconductor material. The depleted volume is created typically by a reverse biased pn junction, i.e. by a reverse biased diode. Radiation entering the semiconductor material creates electron hole pairs. The electric field in the depleted volume separates the electron hole pairs. In neutral semiconductor material the electron hole pairs recombine unless the minority carriers (holes in n type semiconductor material and electrons in p type material) are able to diffuse to the border of the depleted volume. The charge type that is measured will be hereinafter referred to as the signal charges and the opposite charge type as the secondary charges. The area of the semiconductor radiation detector containing the radiation detecting pixels (one or more) is hereinafter referred to as the active area. The ability of the semiconductor radiation detector to detect deeply penetrating radiation like energetic X rays and gamma rays depends on the semiconductor material and on the thickness of the depleted semiconductor volume.
The maximum electric field value in a pn diode is always at the pn junction and the electric field is the smaller the bigger is the distance from the junction. In planar detectors the junction is always close to the surface of the detector. The electric field in a planar detector is thus parallel to the surface normal of the detector and it is a monotonously decaying function of the depth. The direction parallel to the surface normal is herein after referred to as the vertical direction and the two directions on a plane parallel to the front and back surface of the detector are herein after referred to as the two horizontal directions. The maximum thickness of the depleted volume in planar semiconductor radiation detectors is determined by the electric break down field. This means that the planar semiconductor detectors have an upper detection efficiency limit for deeply penetrating radiation. Another problem in planar detectors is that one has to use very high voltages to create a thick depletion region. This is due to the fact that the depletion voltage is proportional to the square of the depletion region thickness. Yet another problem in thick planar detectors is horizontal spreading of the signal charge, which hampers spatial resolution.
In order to solve the aforesaid problems of the planar detectors 3D detectors have been introduced. U.S. Pat. Nos. 5,889,313 and 6,204,087 disclose a 3D detector that has highly doped n and p type 3D electrodes penetrating through a semiconductor substrate. The electric field inside the 3D semiconductor has a horizontal component but there is essentially no vertical component. The depletion voltage of the substrate is thus independent of the substrate thickness. The distance between the 3D electrodes can be made much smaller than the thickness of the semiconductor substrate and thus the depletion voltage of the 3D detector can be very small. The horizontal spreading of the signal charge is prevented due to the horizontal electric field component. The problem associated with this 3D detector is that in case of thick substrates the capacitance of the signal charge collecting 3D electrodes is relatively high due to the large surface area of the 3D electrodes and due to the small distance between the 3D electrodes. In thick substrates the sensitivity of the aforesaid 3D detector to measure the amount of the signal charge is thus deteriorated.
U.S. Pat. No. 5,981,988 discloses a 3D charge coupled device (CCD) that comprises 3D gates that penetrate through the substrate. The gates are made of conducting material and between the gates and the semiconductor substrate there is an isolator layer. The electric field in the 3D-CCD has a horizontal but essentially no vertical component enabling the depletion of a thick substrate with a small voltage and preventing horizontal spreading of the signal charge. The signal charge is collected around the 3D gates during the integration phase. During the signal charge transport phase the signal charge is trans-ported inside the substrate on a plane parallel to the front and back surfaces of the detector. If the radiation is detected continuously the 3D-CCD is, however, prone to smear which weakens the image quality. The smear effect is resulted during the signal charge transport phase because the signal charge collection cannot be turned off. Another problem in a thick 3D-CCD is the transportation of the signal charge from a location around the 3D-gate to a location on the surface of the device where the signal charge is measured. Yet another problem is that efficient collection of the secondary charge is difficult to perform.
PCT/FI2005/050148 discloses a 3D semiconductor detector that has 3D elements of one conductivity type protruding into a substrate of opposite conductivity type. The 3D elements and the substrate are connected to a voltage source, which is used to fully deplete the 3D elements and preferably the substrate too. The electric field inside the detector has due to the 3D elements a horizontal component. If in addition a plane that is parallel to the front and back surfaces of the detector and that is cutting the 3D elements and the substrate contains an equal amount of n and p type doping atoms the resulting vertical electric field component is constant. Due to the constant vertical field there is no limit for the thickness of the detector. The vertical electric field component is used to transport the signal and the secondary charges to the front and back surface of the detector. This 3D detector does not suffer from smear or from horizontal spreading of the signal charge and it has a small capacitance. The manufacturing of such a 3D detector is, however, problematic since it is difficult to precisely control the doping atom distribution inside the 3D detector. First of all, the substrate doping has to be known precisely. Secondly, the shapes of the 3D elements have to be controlled and known precisely which necessitates very accurate processing. Thirdly, the doping concentration of the semiconductor material forming the 3D elements has to be controlled and known precisely. If the manufacturing is not controlled precisely the vertical electric field component will not be constant. The result of this is that the thickness of the detector is limited.
In power electronics the voltage handling capability of a power device is a crucial parameter. The voltage handling capability of semiconductor power devices is, however, too low for many applications and thus non semiconductor based devices like vacuum tubes has to be used instead. The vacuum tubes are, however, bulky, expensive and they suffer from aging effects unlike the semiconductor based power devices. The voltage handling capability of semiconductor power devices is determined by the break down electric field. This fact applies to planar semiconductor power devices and to semiconductor power devices based on the 3D electrodes or 3D gates. This fact applies also to semiconductor power devices based on the 3D elements presented in PCT/FI2005/050148, unless the device is manufactured with extra ordinary precision. Another problem in planar semiconductor power devices is the horizontal spreading of the signal charge. Due to the horizontal spreading of the signal charge adjacent power elements, i.e. e.g. power transistors, have to be spaced far apart on the power chip surface. Unless mixing of signals belonging to adjacent power elements is resulted in.
Based on the above reasoning it is desired to have an improved semiconductor apparatus. The improvement can comprise one or more of the following advantages: the apparatus is easier to manufacture, it can have any desired thickness, it has a low capacitance, it is immune to smear, wherein horizontal spreading of the signal charge is prevented, it can withstand very high voltages and the maximum electric field does not depend on the thickness of the device.